Existing methods for applying tone scales to digital color images include applying the tone scale to each of the color channels independently, applying the tone scale to the neutral channel and preserving the original pixel color difference signal, or applying the tone scale to just the low frequency image. These methods of tone scale application alter the sharpness and appearance of the color in processed images, and the latter can produce displeasing artifacts.
Traditional methods of applying a tone scale function to a digital color image modify the apparent sharpness of the image because the tone scale modifies the amplitudes of high frequency detail information. This phenomenon occurs when the tone scale function is applied to each of the color channels independently, or when the tone scale function is applied to the neutral channel and original pixel color difference signals are preserved.
In an effort to apply a tone scale function to a digital image without distorting the detail information, in U.S. Pat. No. 5,012,333, Lee et al. proposed separating the image into a high frequency and a low frequency image by using FIR filters. The tone scale function is then applied to only the low frequency image, and the high frequency image is added back to the tone scaled low frequency image.
Also, in U.S. Pat. No. 5,454,044, Nakajima suggests modifying the image contrast by the formula EQU Sproc=Sorg+f(Sus).
The low frequency image (Sus) is passed through function f( ) which is a monotonically decreasing function. This signal is added to the original (Sorg) to create the processed image Sproc.
Both of these methods preserve the high frequencies of the image, but both may lead to the creation of unsharp mask type artifacts (overshoot and undershoot) in the neighborhood of large edges (characteristic of occlusion boundaries or dark shadows).
Although the presently known and used methods for adjusting the tone scale of a digital image are satisfactory, they include the above-described drawbacks. The present invention permits the application of a tone scale function to a digital image which adjusts the macro contrast of the image, preserves the high frequency detail information, and prevents artifacts in the neighborhood of large edges.